Pyrolytic beryllia



United States Patent O U.S. Cl. 117-106 1 Claim ABSTRACT F THEDISCLOSURE According to the invention BeO is deposited by vaporizing thebasic formate of beryllium, heating the vapors to a temperature abovetheir decomposition temperature in a region containing a surface heatedto Z50-600 C. whereupon dense BeO is deposited on the surface.

This invention relates to a process of preparing beryllia by thepyro'lytic process and to the pyrolytic beryllia so produced.

Among the objects of the invention is to provide a process of preparinghard, dense and thick beryllia deposits.

Among other objects of the invention is to provide a relatively lowtemperature process for depositing hard, dense beryllia on a substrate.

Among still other objects of the invention is to provide an improvedprocess for the production of pyrolytic beryllia.

Processes for depositing metal oxides by chemical vapor pyrolysisprocesses are known and may be classed generally into one of thefollowing categories.

(1) Deposition by steam hydrolysis of certain metal halides.

(2) Deposition by oxidation of a metal halide with oxygen.

(3) Deposition by thermal degradation of a volatile metal compoundcontaining chemically bonded oxygen.

The recommended deposition temperatures for the above reactions arequite high. Thus, Powell et al. in Vapor Plating, John Wiley & Sons (NewYork 1955), recommend temperatures of the order of 1000 C. for preparingoxide coatings by the steam hydrolysis ofthe halides. Again,Schlossberger et al. in WADC Technical Report 59-363, Research onPyrolytic Deposition of Thin Films, recommend a temperature for thepreparation of oxide tilms by thermal degradation of compoundscontaining oxygen (such as ethyl silicate), of l200 C. Others dealingwith these processes have generally noted that temperatures ofapproximately 1000 C. or above are required `for the preparation of eventhin films.

This invention is based on the discovery that thick, dense, hard,deposits of beryllia can be obtained by the pyrolysis of an organicadduct of beryllium at a relatively low temperature of 300-600 C.

The deposits can be obtained of any desired thickness. The quality ofthe deposit is improved over deposits obtained at 1000 C., for example,because the total stress developed during deposition is less at thelower temperatures.

An organic adduct of beryllium may be defined as a compound in which theberyllium ion and the organic radical are held together by means ofmetal to oxygen to carbon bond, thus differing from organo-metalliccompounds in which a metal to carbon bond exists. A satisfactory organicadduct is the basic formate, Be40 (HCOzle- A process 'for making thebasic organic adducts of beryllium (glucinium) with formic and otherfatty acids is disclosed in Comptes Rendu Vo. 134,772 (1902).

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The process of the invention is particularly useful for providing BeOfilm coatings for the oxidation protection of metals, particularly therefractive metals such as tungsten and molybdenum and for the oxidationprotection of certain non-metals such as plastics, carbon (graphite),etc. The process is also useful for the production of lightweightimpervious structures having a core of foamed or porous material and acoating of the dense pyrolytic BeO. Continuous refractory filaments maybe prepared with a core of boron or nickel and a coating of BeO by thisprocess.

The process also has `applications in the production of complex shapesby selectively depositing the BeO over unmasked portions of a maskedsubstrate.

Thus, the substrate on which the deposit is 4formed can be any materialcapable of withstanding temperatures of about 250 or more and can be ofany shape. Coherent deposits are formed on a substrate of beryllia orother ceramic oxide as well as on the surfaces of refractory metals,plastics, etc.

In the process, the basic formate of beryllia is volatilized(sublimated) and is brought into contact with a preheated substrate in aclosed chamber at sub-atmospheric, atmospheric or superatmosphericpressure. The substrate to be coated is heated to about 250600 C. oreven more if desired. One or both of the substrate and the vapor shouldbe heated to the decomposition temperature of the vapor at the surfacewhere the BeO is to be deposited which is between about 30G-600depending somewhat on the pressure. Lower rates of deposition areobtained at the lower temperatures.

=In the drawing:

FIG. l shows an apparatus suitable for the deposition of beryllia by theprocess of the invention.

FIG. 2 is a photomicrograph (133 of a cross-section of a deposit ofberyllia on a ceramic beryllia substrate made according to the process.

The apparatus of FIG. 1 shows a bell jar 10 fitted on a suitable base 11which is provided with a suitable pipe connection 12 for exhausting the-bell jar 10. A ceramic pedestal 13 is provided to hold a beryllia plate14 which in turn is provided with a heater means 1S connected to theoutside through leads 16. A vaporizing chamber 17 for the organic adductof beryllium is positioned on the heater 15 and beryllia plate 14. Thevaporizing chamber 17 is provided with openings permitting the vapor toescape into the chamber means 20 which is adapted to surround asubstrate to-be-coated 21. Heating means 22 for the substrate havingleads 23 through the base 11 are provided. Thermocouples 24 and 25 tothe vaporizing chamber 17 and deposition chamber respectively areprovided.

The following example is given in detail to further illustrate how thisinvention may be carried out in practice. It is to be understood thatthe specific details given in the examples are not to be considered aslimiting the scope of the invention.

EXAMPLE l The apparatus as shown in FIG. 1 is employed with the basicformate of beryllium added to vessel 17 and with a slab of ceramicallysintered beryllia as the substrate 21. The basic formate compound.sublimes at 185 C. but in this case the bell jar is exhausted to apressure of 30-35 mm. of Hg whereupon the basic formate begins tosublime at about C. The Be() substrate 21 is heated to about 400 C. byheater 22. After about l0 minutes during which the basic formatecompound is heated to 200 C., a deposit of BeO approximately 25 mm. inthickness is retained on the substrate. The deposit is similar to thatshown in FIG. 2, the White portion being the deposit and the lowerdarker portion being the original substrate. It will be noted that thereis a substantially complete absence of pores in the deposited Be()whereas the sintered substrate shows its porous structure.

Beryllia can be deposited on metal or other surfaces in the same way.When the substrate is graphite or other form of carbon it is preferredto initiate the process in an inert atmosphere.

We claim:

1. The process of forming dense beryllia layers comprising:

providing the basic formate of beryllium,

heating the basic formate of beryllium to form vapors of the same,

4. heating said vapors to a temperature above the decompositiontemperature thereof in the region adjacent a surface heated to Z50-600C. whereby to deposit beryllia on said surface.

References Cited Powell et al., Vapor Deposition, 1966i, pp. 402 and 403relied upon.

ALFRED L. LEAVI'IT, Primary Examiner A. GO'LIAN, Assistant Examiner

